1. Field of the Invention
The present invention relates to pliable refractory metal carriers on which a catalyst composition may be coated to provide conformable catalyst members. More particularly, the present invention relates to refractory metal carriers which may be coated with a catalytic composition and then bent to enable insertion of the resulting conformable catalyst into curved conduits and the like, for example, into curved exhaust manifolds or exhaust pipes of gasoline or diesel internal combustion engines.
2. Related Art
The provision of catalysts to treat exhaust gases, such as exhaust gases of internal combustion engines, is, of course, well known in the art. Typically, such catalysts comprise a rigid carrier, such as the so-called “honeycomb”-type carrier, comprising a ceramic-like substance, such as cordierite or mullite, usually of circular or oval cylindrical shape and having a plurality of fine gas flow passages extending from and through the front to the rear face thereof. The walls of these gas flow passages are coated with a catalytic material such as one comprised of a dried, calcined coating of fine particulate refractory metal oxide, e.g., activated alumina, on which is dispersed one or more catalytic metal components. The latter may be, for example, one or more of platinum, platinum plus rhodium, and one or more multi-valent base metal oxides such as oxides of cobalt, nickel, iron or manganese. It is also known in the art to use a carrier comprised of a pair of overlying refractory metal strips, such as stainless steel strips, one flat and one corrugated. The overlying strips are tightly wound into a cylinder so that the alternating corrugated and flat metal strips form a plurality of fine gas flow passages extending from and through the front to the rear face of the metal support. A catalytic material such as described above may be coated onto these gas flow passages, e.g., from an aqueous slurry of the particles, dried and calcined. In either case, whether extruded from a ceramic-like material or fashioned from tightly spiral-wound strips of flat and corrugated metal, the resulting carrier body is rigid. In order to provide sufficient catalytic material to effectuate purification of the exhaust stream being treated, such rigid catalyst members are typically of significantly larger diameter than the exhaust stream conduits in which they are placed. Therefore, it is well known in the art to encase such rigid catalyst members within a canister, such as a stainless steel canister, adding inlet and outlet ends which are of truncated conical configuration, the larger base of the truncated conical ends being attached to a cylindrical housing which contains the rigid catalyst member. The smaller ends of the truncated conical end sections face away from the rigid catalyst member and are sized to be conveniently connected, respectively, to inlet and outlet conduits which flow the exhaust stream into and carry it from the rigid catalyst member.
The foregoing construction is old and well known in the art. It is also known in the art that the application of such catalytic materials to metal substrates is enhanced by the application of an intermetallic compound to the metal substrate as an intermediate coating between the metal and a catalytically active layer. For example, such an arrangement is shown in U.S. Pat. No. 5,204,302, issued Apr. 20, 1993 to I. V. Gorynin et al, and entitled “Catalyst Composition and a Method For Its Preparation” (“the '302 Patent”). The '302 Patent discloses a multi-layered catalyst material supported on a metal substrate. The metal substrate (column 4, lines 64-68) may be any thermally stable metal including stainless steel and low alloy steel. As illustrated in FIG. 1 of the Patent and described at column 4, line 32 et seq, a flame spraying or plasma spraying apparatus (FIG. 2 and column 5, line 32 et seq) is used to apply an adhesive sublayer 12 to metal substrate 11, which is shown in solid cross section as a dense (solid) plate-like structure. Adhesive sublayer 12 contains a self-bonding intermetallic compound formed from any one of a number of metal pairings, including aluminum and nickel, as described at column 5, lines 1-6 of the '302 Patent. A catalytically active layer 14 (FIG. 1) is sprayed atop the sublayer 12 and has a gradient composition with an increasing content of catalytically active material as one proceeds away from the interface (column 5, lines 7-24). The catalytically active layer can be alumina, preferably gamma-alumina, and may further include specified metal oxide stabilizers such as CaO, Cr2O3, etc., and metal oxide catalytic materials such as ZrO2, Ce2O3, etc. A porous layer 18 (FIG. 1 and column 5, lines 25-32) contains some catalytically active components and transition metal oxides as decomposition products of pore-forming compounds such as MnCO3, Na2CO3, etc. An optional activator coating 19 may be applied onto the porous layer, preferably by magnetron sputtering (see column 4, lines 56-63 and column 8, lines 24 et seq).
So-called “metal foams” and their use as a substrate or carrier for catalysts used in the treatment of automotive exhaust gases are also known in the art. For example, U.S. Pat. No. 3,111,396 to Ball, dated Nov. 19, 1963, discloses a method for making a porous “metal foam”. Essentially, the method comprises forming a porous organic structure such as a mesh, cloth, or a cured foam structure such as an open pore sponge, impregnating the structure with a fluid suspension of powdered metal in a liquid vehicle, and drying and heating the impregnated structure to remove the liquid vehicle and then further heating the organic structure to decompose it and to sinter the metal powder into a continuous form. The resulting metallic structure, while not foamed during the manufacturing process, is nevertheless described as foamed because its ultimate structure resembles that of a foamed material.
SAE (Society of Automotive Engineers) Technical Paper 971032, entitled A New Catalyst Support Structure For Automotive Catalytic Converters by Arun D. Jatkar, was presented at the International Congress and Exposition, Detroit, Mich., Feb. 24-27, 1997. This Paper discloses the use of metal foams as a substrate for automotive catalysts and notes that foams made from FeCrAlloy and ALFA-IV® ferritic stainless steel powders were said to be successful, at least in preliminary tests, for use as substrates for automotive catalysts. A ceramic washcoat having a precious metal loading was deposited onto disks of ALFA-IV® metal foam produced by Astro Met, Inc. The washcoat comprised gamma-alumina and cerium oxide on which platinum and rhodium in a ratio of 4:1 were dispersed to provide a loading of 40 grams of the precious metal per cubic foot of the foam-supported catalyst. Such catalyzed substrates were said to be effective in treating hydrocarbon emissions.
In an article entitled “Catalysts Based On Foam Metals”, published in Journal of Advanced Materials, 1994, 1(5) 471-476, Pestryakov et al suggest the use of foamed metal as a carrier substrate for catalytic materials for the catalytic neutralization of exhaust gases of car engines. The use of an intermediate layer of high surface area alumina between the metallic foam and the catalytic material is recommended, by direct deposition on the foam carrier. In addition to increasing the surface area of the substrate, the alumina is also credited with protecting the surface of the substrate against corrosion.
SAE Paper 962473 by Reck et al of EMITECH, GmbH, entitled “Metallic Substrates and Hot Tubes For Catalytic Converters in Passenger Cars, Two- and Three-Wheelers” addresses the use of catalytic converters and hot tubes to treat the exhaust of scooters and motorcycles, especially those having two-stroke engines.
Wire mesh carriers for catalytic materials are commercially available and comprise wire that has been plasma spray coated to form a rough surface thereon to improve the adherence of a catalytic material deposited thereon.